Preparation of molded articles



United States Patent 3,093,494 PREPARATION OF MOLDED ARTICLES Robert C. Hedlund and Myron Kin, Midland, Mich., as-

signors to Dow Corning Corporation, Midland, Mich, a corporation of Michigan e No Drawing. Filed June 12, 1961, Ser. No. 116,299

4.Claims. (Cl. 10638.2)

The present invention relates to the field of precision casting of metals.

In recent years several methods have been devised for precision casting of metals. One of the most successful methods has been the so called Croning process in which the face of the mold is composed of an aggregate bonded with an organic resin such as a phenolic resin. By using this method relatively good precision is obtained which minimizes the amount of machining necessary. However, for extremely high precision this method is inapplicable because the organic resin binder gases when the molten metal is poured into the mold. This gassing gives a defective metal surface which cannot be tolerated when extremely high precision is needed in the casting. At-

tempts have been made to avoid this difiiculty by employing al-kyl silicates. However, it has been found that these materials sufier from two serious disadvantages; The first is that the alkylsilicates produce gas on contact with the molten metal unless the mold has been previously heated to temperatures of 1800 F; or above. To fire at such temperatures is both expensive and time-consuming. Furthermore, it has been found that attempts to prepare silicate molds by the commonly employed pamming technique results in a cracking of the mold during'curing. Consequently, alkyl silicates are not suitable.

Attempts have also been made to employ alkali metal silicates and al-kali metal salts of silan-ols as the bonding agent for the aggregate. However, these materials suifer from the serious disadvantage of forming hard crusts on the surface of the metal. This is caused by the vitrification of the mold surface when in contact with the molten metal. As a result it is extremely d-ifficult to remove the mold from the metal surface. Consequently, this is a time-consuming and expensive method.

a It is the object of this invention to avoid the above difficulties. The process described below has the following advantages over the prior art methods:

()1) it employs a water soluble material.

(2) The molds can be for-med by a simple ramming technique.

(3) The molded article'can be cured without cracking and they can be used in the conventional ovens used in iioundries. (4) The molded articles will not gas in contact with molten metal. (5) The mold is easily removed from the surface of the cast metal.

These advantages provide a cheaper method for precision casting of metals.

This invention relates to a method of preparing molded,

employed is suilicient to .bond the latter into a non-r crumblingandnon-slumping article'when the mixture is shaped by a ramming process and (2) in amount such that the residual silica after firing is at least 0.4 percent 3,093,494 Patented June 11, 1963 7 by weight based on the weight of the aggregate, thereafter l to the desired strength ator below 700 F. This means.

I pound varies with the particle size of the aggregate.

forming the mixture into the desired shape by ram molding, drying the molded article to remove most of the water, and thereafter thing the shaped article at a temperature not greater than 700 F. until the hydrolyzate is reduced to silica whereby a unitary bonded article having a compressive strength of at least 400 psi. is obtained.

In the practice of this invention the silane is mixed with water and agitated until it hydrolyzes to give a homogeneous solution. The solution is facilitated by employing a small amount of a mild acid such as acetic, propionic and the like. These solutions are stable for matters of several weeks and are ready for use in the process of this invention.

The aqueous dispersion thus formed is then mixed in any suitable manner with any desired foundry aggregate. The relative proportions of water and organosilicon compound employed should be such that the mixture is noncrnmbling and non-slumping. This is to insure that the aggregate can be satisfactorily formed in a conventional ramming process. If the amount of water is too little,

the formed article crumbles upon removal from the pat- The amount of organosilicon compound employed must be such that the residual silica after tiring will be at least .4 percent by weight based on the weight of the aggregate. Here again the optimum amount of organosilicon com- For coarser particles less organosilicon compound is needed, and for more finely divided aggregate a larger proportion of organosilicon compound is needed. In all cases the amount used should be ,suflicient to produce a tired article of at least 400 p.s.i. compressive strength. The optimum proportions can readily be determined by observing the compressive strength of the fired objects. Obviously for reasons of economy the amount of silane should be kept as low as will give the desired strength. In general, the organosilicon compound will be in minor amount relative to the Weight of the aggregate.

Ai 'ter the aqueous dispersion has been mixed with the aggregate, the mixture is formed by conventional ramming technique and the formed article is then allowed to dry to remove most of the water. If desired, the drying can be facilitated by heating at a temperature of C. or above, however, they can also be allowed to air dry.

The dried articles are then fired at temperatures of below 700 F. until the organosilicon compound is reduced to silica. At that point the article stops losing weight. In general, heating at temperatures of 600 to 700 F. for 8 hours is sufiicient. However, lower temperatures may be employed for longer times if desired;

' Objects made in accordance with this invention have a compressive strength of 400 psi or above. These strengths are needed in order to prepare molds of suflicient durability to be satisfactory in the precision casting of metals.

Forthe purpose of this invention any foundry aggregate suitable for use in the formation of sand molds for pre 3 EXAMPLE 1 In each of the mixtures shown below the ethyltrimethoxysilane is hydrolyzed by mixing it with a .1 percent solution of acetic acid in water. The mixture was stirred until the solution was complete and the solution was then mixed with the zirconium silicate in a Kitchen Aid mixer. The resulting mixtures were then made into standard 2 inch cylinders on the Dietert sand rammer. The cylinders so formed were dried at 105 C. for 8 hours and then heated at 600 F. in air for 8 hours. This oxidized the ethyl groups in the residual ethyl siloxane to produce SiO After cooling to room temperature, the compressive strength of each cylinder was determined. The results are shown below.

The zirconium silicate sand employed had a particle size distribution as follows:

None larger than 420 microns 4.06% larger than 250 microns 34.8% larger than 149 microns 50.6% larger than 105 microns 7.82% larger than 74 microns .37% larger than 65 microns 98% smaller than 65 microns The zirconium silicate flour employed at a particle size as follows:

None larger than 250 microns .013% larger than 149 microns .005 larger than 105 microns .075 larger than 74 microns .400% larger than 65 microns 99.400% smaller than 65 microns Table I Zirconium Zirconium Ethyltri- Mixture silicate silicate methoxy- Water, g

No. sand, g. fiour, g. silane, g.

The SiO content and the compressive strength of the articles made from each of these mixtures are shown in the table below:

In each of the samples shown below ethyltrimethoxysilane was hydrolyzed by mixing it with an aqueous solution of .1 percent by weight acetic acid. The aqueous solution of the hydrolyzate so formed was then mixed with a mixture of 420 g. of No. 48 kyanite and 180 g. of No. 100 kyanite. The amount of the aqueous solution was varied so as to give the percent by weight silica based on the weight of the kyanite shown in the table below.

After mixing the solution with the kyanite the mixture was formed and 2 inch cores on a Dietert sand rammer. The cores were then dried for 8 hours at 105 C. and then fired in air for 8 hours at 660 F. to oxidize the ethylsiloxane to SiO After cooling the cores were tested for compression strength. The results are shown in Table III below.

The particle size distribution of the No. 48 kyanite is as follows.

.45 larger than 420 microns 3.22% larger than 250 microns 10.55% larger than 149 microns 15.48% larger than microns 16.95% larger than 74 microns 8.88% larger than 65 microns 44.35% smaller than 65 microns Table III Compressive strength, Percent by wt. SiO after firing: p.s.i. 1. 175

EXAMPLE 3 25 g. of ethyl trimethoxysilane was mixed with 25 g. of an aqueous solution containing .1 percent by weight acetic acid. The mixture was shaken until a homogeneous solution was obtained and then mixed with a mixture of 600 g. of No. 70 olivine sand and 200 g. of No. olivine sand. The mixture was formed in a ram mold and the molded article dried at 105 C. for 8 hours. The resulting anticle was then heated in air at 660 F. for 8 hours. The room temperature compressive strength was 502 p.s.i. and the article contained 1.25 percent S10 based on the weight of the total olivine.

The experiment was repeated using 40 g. of ethyltrimethoxysilane, 40 g. of .1 percent aqueous acetic acid, 450 g. of -No. 70 olivine sand and 350 g. of N0. 150 olivine sand. The resulting fired article contained 2.0 percent by weight Si0 based upon the total weight of the olivine and had a compressive strength of 995 p.s.i.

The No. 70 olivine sand had the following particle size distribution 3.7% larger than 420 microns 49.15% larger than 250 microns 33.4% larger than 149 microns 5.69% larger than 105 microns 2.8% larger than 74 microns .9% larger than 65 microns 4.2% smaller than 65 microns The olivine sand No. 150 had the following particle size:

None larger than 250 microns 1.7% larger than 149 microns 16.7% larger than 105 microns 62.1% larger than 74 microns 6.6% larger than 65 microns 12.6% smaller than 65 microns EXAMPLE 4 Employing the procedure of Example 3 a fired article was prepared from a mixture of 700 g. of 100 mesh alumina, 200 g. of 325 mesh alumina, 25 g. of ethyltrimeth'oxtysilane and 25 g. of .1 percent aqueous acetic acid. The resulting fired article had a compressive strength of 659 p.s.i. and contained 1.1 percent by weight S10 based upon the weight of the total alumina.

This experiment was repeated employing 700 g. of 100 mesh alumina, 200 g. of 325 mesh alumina, 40 g. of ethyltrimethoxysilane and 40 g. of .1 percent aqueous acetic acid. The resulting tired article had a compressive strength of 1231 p.s.i. and contained 1.78 percent by weight Si based upon the weight of the alumina.

EXAMPLE 5 48.1 g. of 'ethyltrieth'oxysilane was mixed with 48.1 g. of a .1 aqueous acetic acid. The mixture was shaken overnight to give a homogeneous solution. This solution was then mixed with a mixture of 420 g. of No. 48 kyanite and 180 g. of No. 100 kyanite. The mixture was formed into a cylinder in the Dietert sand rammer and dried 8 hours at 105 C. and then fired in air 8 hours at 660 F. to give an article containing 2.5 percent SiO based on the weight of the kyanite. The resulting article had a compressive strength of 636 p.s.i.

EXAMPLE 6 Equivalent results are obtained when 52.5 g. of propyltrimethoxysilane is employed in the process of Example 2. The resulting article would contain 3.15 percent by weight SiO based upon the weight of the kyanite.

That which is claimed is:

1. A method of preparing articles suitable for use as molds and cores in the forming of cast metal articles to close tolerances which comprises mixing an aqueous dispersion of a hydrolyzate of a silane of the formula RSi(OR') where R is an alkyl radical of from 2 to 3 inclusive carbon atoms and R is an alkyl radical of from 1 to 2 inclusive carbon atoms with a foundry aggregate in amount such that (l) the amount of water in the aggregate is sutficient to bond the aggregate into a noncrumbling and non-slumping article when the mixture is shaped by a ramming process and (2) the amount of silane employed is such that the residual silica after firing is at least .4 percent by weight based on the Weight of the aggregate, thereafter forming the mixture into the desired shape by ram molding, drying the molded article to remove most of the water and thereafter heating the shaped article at a temperature not greater than 700 F. until the hydrolyzate is reduced to silica, whereby a unitary bonded article having a compressive strength of at least 400 psi. is obtained.

2. The method of claim 1 wherein the silane is ethyltrimeth-oxysilane.

3. The method according to claim 1 wherein the silane is ethyltriethoxysilane.

4. The method according to claim 1 wherein the silane is propyltrimethoxysilane.

References Cited in the file of this patent UNITED STATES PATENTS 2,967,338 Cooper Jan. 10, 1961 FOREIGN PATENTS 575,734 Great Britain June 13, 1946 

1. A METHOD OF PREPARING ARTICLES SUITABLE FOR USE AS MOLDS AND CORES IN THE FORMING OF CAST METAL ARTICLES TO CLOSE TOLERANCES WHICH COMPRISES MIXING AN AQUEOUS DISPERSION OF A HYDROLYZATE OF A SILANE OF THE FORMULA RSI(OR'')3 WHERE R IS AN ALKYL RADICAL OF FROM 2 TO 3 INCLUSIVE CARBON ATOMS AND R'' IS AN ALKYL RADICAL OF FROM 1 TO 2 INCLUSIVE CARBON ATOMS WITH A FOUNDRY AGGREGATE IN AMOUNT SUCH THAT (1) THE AMOUNT OF WATER IN THE AGGREGATE IS SUFFICIENT TO BOND THE AGGREGATE INTO A NONCRUMBLING AND NON-SLUMPING ARTICLE WHERE THE MIXTURE IS SHAPED BY A RAMMING PROCESS AND (2) THE AMOUNT OF SILANE EMPLOYED IN SUCH THAT THE RESIDUAL SILICA AFTER FIRING IS AT LEAST 4 PERCENT BY WEIGHT BASED ON THE WEIGHT OF THE AGGREGATE, THEREAFTE FORMING THE MIXTURE INTO THE DESIRED SHAPE BY RAM MOLDING, DRYING THE MOLDED ARTICLE TO REMOVE MOST OF THE WATER AND THEREAFTER HEATING THE SHAPE ARTICLE AT A TEMPERATURE NOT GREATER THAN 700*F. UNTIL THE HYDROLYZATE IS REDUCED TO SILICA, WHEREBY A UNITARY BONDED ARTICLE HAVING A COMPRESSIVE STRENGTH OF AT LEAST 400 P.S.I. IS OBTAINED. 